Reading the results tables of a daylight assessment

Daylight assessment results tables look forbidding and are mostly arithmetic. Three measures do almost all the work: vertical sky component for the amount of sky a window sees, no sky line for how far daylight reaches into a room, and annual probable sunlight hours for direct sun. Once you know what each one describes, and the difference between an absolute value and a proportional change, a fifty page appendix becomes readable in twenty minutes.

The guidance behind them is BRE 209, Site Layout Planning for Daylight and Sunlight, reissued in 2022. It calls its numbers guidelines rather than standards and says in terms that they should be applied flexibly. That sentence is worth quoting in any statement, because the tables are routinely treated as pass or fail.

What each measure describes

Vertical sky component is the proportion of light from an overcast sky reaching a point at the centre of a window, expressed as a percentage of what an unobstructed vertical surface would receive. The theoretical maximum for a vertical wall with nothing in front of it is just under 40 per cent.

No sky line, sometimes called daylight distribution, is a plan measure rather than a window measure. It divides a room into the area that can see the sky from the working plane and the area that cannot, and the assessment reports how that lit area changes.

Annual probable sunlight hours measures direct sun rather than diffuse light. It is reported as a percentage of the total annual hours of probable sunlight available at the location, with a separate figure for the winter months between late September and late March.

Rooms are assessed by use. Living rooms, dining rooms and kitchens carry more weight than bedrooms, and bathrooms, hallways, stores and circulation space are not assessed at all. If a table treats a bedroom as the headline failure in a flat with a compliant living room, read the room schedule before accepting the conclusion. The relationship between the two main measures is set out more fully in vertical sky component and no sky line compared.

Reading a vertical sky component table

A typical table gives each window a reference, the existing value, the proposed value, the absolute difference and the ratio of proposed to existing.

But two numbers matter most. The guideline absolute value is 27 per cent, above which a window is treated as receiving enough skylight whatever the change. The guideline ratio is 0.8, so a window retaining at least 80 per cent of its former value is treated as unlikely to be noticeably affected.

Both tests have to fail before the guidance suggests occupants would notice. A window falling from 32 to 28 per cent has dropped below the ratio in some arrangements but stays above the absolute value, and the assessment should say so rather than colouring the cell red.

Watch for window references that do not tie to a drawing. A table listing W1 to W46 without an annotated elevation is not checkable, and asking for the elevation is a reasonable first request on any objection.

No sky line and room by room effects

The no sky line results appear as areas in square metres, existing and proposed, with a ratio. The same 0.8 guideline applies.

This measure catches something the window test misses. A room can hold an acceptable vertical sky component at its window and still lose daylight at the back, particularly where the room is deep, where a balcony overhangs the window, or where the obstruction is close and wide. Deep plan flats behind a single window are the classic case.

Read it alongside the room dimensions. A 12 square metre bedroom losing 30 per cent of its lit area is a different proposition from a 30 square metre living and dining space losing the same proportion at the far end, where the kitchen sits.

Sunlight hours and orientation

Only windows facing within 90 degrees of due south are assessed for sunlight. Anything facing north is excluded, which frequently explains why a scheme with a serious overshadowing complaint shows almost no sunlight results.

The guidelines are an annual figure of at least 25 per cent of annual probable sunlight hours, with at least 5 per cent falling in the winter months. Where a window falls below both, the guidance then looks at whether the retained value is less than 0.8 times the former value and whether the annual loss exceeds 4 per cent of annual probable sunlight hours.

Winter sunlight is where schemes in dense areas usually fail, because a low winter sun is blocked by obstructions that a summer sun clears. A table showing a comfortable annual figure and a winter figure of 2 per cent is describing a room that will be sunless through the winter half of the year, and that deserves comment even when the annual test passes.

Proportional change against absolute value

This is the distinction that decides most arguments, and it cuts both ways.

The fourth row is the trap. Where a baseline is already low, ratios flatter the scheme, because small absolute losses produce small proportional ones. Applicants lean on the ratio and objectors lean on the absolute value, and an honest assessment reports both with the retained absolute figure stated plainly. Where the starting point is poor, the analysis in assessing daylight where existing conditions are already poor explains what a decision maker can sensibly do with the numbers.

Urban context cuts the other way too. BRE 209 accepts that its numerical guidelines came out of suburban conditions, and it allows alternative targets suited to a dense setting, which several London boroughs have adopted. Applying a 27 per cent expectation to a city centre street is what the guidance itself warns against, a point developed in applying BRE 209 daylight guidance in dense London.

Gardens and amenity areas

Sunlight to open space is a separate test with a simple output. At least half the area of a garden or amenity space should receive at least two hours of sunlight on 21 March.

The result is usually given as a percentage of the area achieving the two hours, existing and proposed, with a plan showing the shaded envelope. Ask which date was modelled, because 21 June flatters every scheme and 21 December condemns most. The equinox is the reference date, and a study running only the summer solstice is not answering the question. Transient shadow studies through the day are a different exercise again, closer to what an Overshadowing Study sets out.

Questions to ask about the model

Six questions expose most of the weaknesses in a set of tables.

1. Was the existing baseline the site as it stands, or as consented, and were other permitted schemes included?
2. How were the internal layouts of neighbouring properties established, and where they were unknown, what assumption replaced them?
3. Were room uses taken from a survey, from estate agent particulars, or assumed?
4. Were balconies, overhangs, reveals and external walkways modelled?
5. What surface reflectances and what transmittance for the glazing were used?
6. Does the window schedule tie to annotated elevations, and does the room schedule tie to plans?

Where answers are missing, say so rather than disputing the numbers. An assessment that cannot be reconstructed from its own appendices carries less weight than one that can.

A Daylight and Sunlight Assessment is a report type where councils expect a suitably qualified author, and professional review before submission is strongly advised. PlanScope provides AI-assisted draft reports for planning applications. Reports should be reviewed before submission. We do not guarantee planning approval. This is general guidance rather than legal advice, so check the position with your local planning authority or a qualified assessor before relying on it.